RE: Bug in loop optimize (TREE stage)

[David Korn <dkorn at pixelpower dot com>]

>-----Original Message-----
>From: Denis Chertykov [mailto:denisc@overta.ru]
>Sent: 26 December 2000 20:06

>"Alexander Popov" <sasho@vip.orbitel.bg> have founded a bug in generic
>GCC code.
>I have tried  avr and x86 ports.
>
>The XXX.rtl file already have a bug.
>
>
>volatile unsigned char *p;
>volatile unsigned char i;
>
>int main(void) {
>
>   do {
>   	i++;
>   } while(p++<(unsigned char *)0xffffffff);
>  /* if 0xffffffff change to 0xfffffffe then no bug */
>   return 0;
>}

  This is undefined behaviour. Since you haven't initialised variable p,
the compiler is free to implement whatever it likes here. In particular,

>.globl main
>	.type	main,@function
>main:
>	movb	i, %al
>	pushl	%ebp
>	incl	%eax
>	movl	%esp, %ebp
>	incl	p
>	movb	%al, i
>	xorl	%eax, %eax
>	popl	%ebp
>	ret

 it is free to arbitrarily decide that the loop condition, being undefined,
will evaluate to FALSE, and so it decided here that the loop body would
run once; then it optimized the loop away altogether.  In short, gcc is
right and your testcase is wrong.


       DaveK
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